Closures often contain an inner liner which is used to form a more complete seal between the closure and a container. The liner may be secured to the closure by the use of glue. A drop of glue is deposited on the inner surface of a closure and the liner is affixed later. In order to make sure that the glue drop is being deposited on the closure properly, it is necessary to check to see if a glue drop is present before attaching a liner. Previously, this has been accomplished by either visual inspection or by the use of an infrared detector, which can be used only when the glue drop is relatively hot when it is deposited on the closure surface.
Principal advantages of the present invention are that visual inspection of the closure is not needed and that there is no need for a temperature gradient between the glue drop and closure surface.